Conventionally, there has been known a mobile communication system capable of accommodating a 2G/3G radio access system and an LTE (Long Term Evolution) radio access system.
With reference to FIG. 25 and FIG. 26, an operation will be described, in which a mobile communication system switches a state (a first communication state) where a mobile station UE#1 performs voice communication (VoIP communication) with respect to a mobile station UE #2 via an LTE radio access system to a state (a second communication state) where the mobile station UE #1 performs voice communication (circuit-switched communication) with respect to the mobile station UE #2 via a 2G/3G radio access system, that is, an operation for realizing “SRVCC (Single Radio Voice Call Continuity)” defined in Non-Patent Document 1 will be described.
In step 1 in FIG. 25, the LTE radio access system (Source E-UTRAN) transmits a switching preparation instruction for the voice communication of the mobile station UE #1 to an enhanced MSC/MGW (Enhanced MSC Server Media Gateway for SRVCC) via a node MME (Mobility Management Entity, Source MME) (steps 3 to 5a in FIG. 26).
In step 2 in FIG. 25, the enhanced MSC/MGW transmits a resource preparation instruction for circuit-switched communication to a mobile switching center MSC (Mobile-service Switching Center, Target MSC) and the 2G/3G radio access system (Target RNC/BS), thereby preparing a resource for the voice communication (circuit-switched communication) of the mobile station UE #1 in an interval between the enhanced MSC/MGW and the 2G/3G radio access system (a switching destination) (steps 5b, 5c, 8a, 8b, and 8c in FIG. 26).
In step 3a in FIG. 25, the enhanced MSC/MGW transmits a switching request of a path of a VoIP media signal and a VoIP control signal to a node SCC AS (Service Centralization and Continuity Application Server) arranged in a home network of the mobile station UE #1 in IMS (IP Multimedia Subsystem) (step 9 in FIG. 26), and the node SCC AS transmits the switching request of the path of the VoIP media signal to the mobile station UE #2.
Furthermore, in step 3b in FIG. 25, the enhanced MSC/MGW notifies the LTE radio access system of the completion of the above-mentioned switching preparation, thereby transmitting a switching instruction from the LTE radio access system to the 2G/3G radio access system to the mobile station UE #1 (steps 12 to 14 in FIG. 26).
As a consequence, the VoIP media signal is switched from a state (a first communication state) where the VoIP media signal is exchanged between the mobile station UE #1 and the mobile station UE #2 via the LTE radio access system, a node S-GW (Serving-Gateway), and a node P-GW (a PDN-Gateway) to a state (a second communication state) where the VoIP media signal is exchanged between the enhanced MSC/MGW and the mobile station UE #2.
Furthermore, the VoIP control signal (SIP signal) is switched from a state (a first communication state) where the VoIP control signal is exchanged between the mobile station UE #1 and the mobile station UE #2 via the LTE radio access system, the node S-GW, the node P-GW, and the IMS to a state (a second communication state) where the VoIP control signal is exchanged between the enhanced MSC/MGW and the mobile station UE #2 via the IMS.
Thus, between the mobile station UE #1 and the enhanced MSC/MGW, a signal (hereinafter, referred to as a “circuit-switched signal”) in circuit-switched communication including circuit-switched data and a control signal is exchanged via the 2G/3G radio access system. Here, the enhanced MSC/MGW is configured to convert the circuit-switched signal and a combination of the VoIP media signal and the VoIP control signal.
In addition, U plane data (hereinafter, referred to as a “packet signal”), other than the above-mentioned VoIP media signal and VoIP control signal, is switched from a state (a first communication state) where the packet signal is exchanged between the mobile station UE #1 and a packet communication network via the LTE radio access system, the node S-GW, and the node P-GW to a state (a second communication state) where the packet signal is exchanged between the mobile station UE #1 and the packet communication network via the 2G/3G radio access system, the node S-GW, and the node P-GW.